Amplifier with modulated power supply voltage

ABSTRACT

The power supply voltage of an amplifier is modulated by a signal having a waveform that corresponds, at least in selective parts, to the waveform of the signal being amplified by the amplifier.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an amplifier that has a powersupply voltage source, the output voltage of which is modulated inaccordance with the signal being amplified by the amplifier.

2. Description of the Prior Art

It is customary in amplifiers, particularly single-ended, push-pullamplifiers, to supply a fixed direct voltage. In the case ofsingle-ended, push-pull amplifiers, the direct voltage power supplyterminals may include a negative voltage terminal below the ground, orreference, voltage and a positive terminal above the reference voltage.The signal being amplified normally swings back and forth about thezero, or reference, point. As a result, in single-ended, push-pullamplifiers, current flows through the amplifying components, usuallytransistors of opposite conductivity type, when the voltage across therespective one of those transistors is high and that transistor isconductive. As a result, there is considerable loss of power and theefficiency of the amplifier circuit is relatively low.

SUMMARY AND OBJECTS OF THE INVENTION

It is an object of the present invention to provide an improvedamplifier circuit of greater efficiency than prior art amplifiercircuits by modulating the power supply voltage appropriately.

According to one aspect of the present invention, an amplifier circuitis provided that includes an input terminal to which an input signal isapplied. The amplifier further includes an output terminal from which anoutput signal is obtained and a load is connected to the outputterminal. The power supply circuit for the amplifier connecting theinput terminal to the output terminal includes means for modulating theoperating voltage supplied to the amplifier. The modulation is inaccordance with the signal being amplified, and may be taken from theinput terminal or the output terminal.

If the amplifier is a push-pull amplifier of the single-ended typehaving both a negative power supply terminal and a positive power supplyterminal, the negative direct voltage from the power supply itself andthe positive voltage from the power supply are both modulated accordingto the signal being amplified. Such modulation may be achieved by atransistor having its collector-emitter circuit connected in seriesbetween the respective source terminal of the power supply, i.e., thepositive terminal and the negative terminal and the corresponding powersupply voltage terminal of the single-ended, push-pull amplifier. Theseries connected transistors in the positive and negative sections ofthe power supply may be controlled by pulse width modulated signals tocause those transistors to conduct current having a value thatcorresponds to the instantaneous value of the amplified signal. Thepulsating currents in both positive and negative sections are smoothedby suitable smoothing filters, and the output voltage of these filtersis actually the direct voltage to operate the single-ended, push-pullamplifier.

The pulse width modulated signals to control the series connectedtransistors can be derived from comparator circuits that compare thesignal being amplified, which may be obtained from across the load ofthe single-ended, push-pull amplifier, with a relatively high frequencysawtooth signal of the appropriate polarity such that for intervals whenone section of the single-ended, push-pull amplifier is operative, theappropriate power supply voltage value will be supplied to it, and whenthe other polarity of the signal is to be amplified, the proper value ofthe power supply voltage will be supplied to the other section of thepush-pull amplifier.

In the case of a normal push-pull amplifier the amplified signal may bepassed through a fullwave rectifier to generate the proper waveform tomodulate the single power supply voltage applied to the push-pullamplifier.

Even if the amplifier is a single-ended amplifier, its efficiency maystill be improved by modulating the power supply voltage by the signalbeing amplified, for example, by obtaining the modulating signal fromthe output of the single-ended amplifier.

Further objects and advantages of the invention will become apparentfrom the following description taken in conjunction with the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a standard single-ended, push-pullamplifier according to the prior art.

FIGS. 2A-2E are waveforms used for explaining the operation of thecircuit in FIG. 1.

FIG. 3 is a schematic circuit diagram of one embodiment of an amplifiercircuit according to the present invention.

FIGS. 4A-4D are waveforms of signals that occur in the operation ofcomparators in the circuit in FIG. 3.

FIGS. 5A-5F are waveforms of signals that occur in the operation of theamplifier circuit in FIG. 3.

FIG. 6 is a graph showing the output voltage versus efficiencycharacteristics of the amplifier circuit of the present invention.

FIG. 7 is a schematic circuit diagram of another embodiment of theinvention.

FIGS. 8A-8F are waveforms of signals that occur in the operation in thecircuit in FIG. 7.

FIG. 9 is a schematic circuit diagram of another embodiment of theinvention.

FIGS. 10A-10D are waveforms of signals that occur in the operation ofthe circuit in FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a single-ended, push-pull amplifier that comprises an NPNtransistor Q₁ and a PNP transistor Q₂. The emitters of the transistorsare connected directly together and the collectors of the transistorsare connected, respectively, to a positive power supply terminal +B anda negative power supply terminal -B. The voltage at the positive powersupply terminal is +V_(CC), and the voltage at the negative power supplyis -V_(CC). The bases of the two transistors Q₁ and Q₂ are connecteddirectly together to an input terminal t₁ to receive an input signalv_(i). The emitters of the transistors are connected to an outputterminal t₂ and a load Z_(L) is connected between this output terminaland ground, which serves as a reference voltage point. An output voltagev_(o) is applied across the load Z_(L) and may be measured at theterminal t₂.

In the operation of the circuit in FIG. 1, the waveform of the outputvoltage v_(o) is illustrated on FIG. 2A as having no DC component. Thepositive peak of the voltage v_(o) is less than the voltage +V_(CC), andthe negative peak of the voltage v_(o) has a magnitude less than themagnitude of the negative power supply voltage -V_(CC). With powersupply voltages of such magnitude, the collector-emitter paths of thetransistors Q₁ and Q₂ are conductive.

As illustrated in FIG. 2B, the collector-emitter voltage across thetransistor Q₁ is v₁. This voltage causes a current i₁, having a waveformshown in FIG. 2C, to flow through the collector-emitter circuit of thetransistor Q₁. In a similar manner, there is a voltage v₂ shown in FIG.2D across the emitter-collector circuit of the transistor Q₂, causing acurrent i₂, that has a waveform shown in FIG. 2E, to flow through thisemitter-collector circuit. As a result, during positive half-cycles ofthe input signal voltage v_(i) applied to the terminal t₁ in FIG. 1, thetransistor Q₁ becomes conductive. During each negative half-cycle of theinput voltage v_(i) the transistor Q₂ becomes conductive so that thecurrents i₁ and i₂ flow through the transistors Q₁ and Q₂ alternatively.It is assumed that push-pull, single-ended amplifiers produce negligibledistortion of the input signal v_(i), and therefore the output signalv_(o) at the terminal t₂ has the same waveform and is in phase with thesignal v_(i).

It is apparent from inspection of FIGS. 2A-2E that, when no currentflows through either of the transistors Q₁ or Q₂, there is nodissipation of energy, even if the voltages across the transistors Q₁and Q₂ are high. However, if a high voltage is applied across theemitter-collector circuit of transistor Q₁ or the collector-emittercircuit of transistor Q₂ when these transistors are conductive, there isa substantial power loss in the respective conductive transistor. As aresult, the efficiency of the amplifier circuit becomes low.

In general a single-ended, push-pull amplifier of the type shown in FIG.1 is more efficient when operated as a class B amplifier than whenoperated as a class A amplifier. The theoretical value of efficiency ofa class B amplifier in proportion to its output amplitude increases asthe output amplitude itself increases and reaches a value of about 78%at the maximum output level P_(o) max. This is illustrated by thestraight line a in the graph in FIG. 6. However, such a percentage ofefficiency is not sufficient.

FIG. 3 shows one embodiment of an amplifier circuit according to thepresent invention utilizing the same single-ended, push-pull amplifiercircuit as shown in FIG. 1. This circuit is designated AMP in FIG. 3,and the components thereof are identified by the same referencecharacters as the corresponding components in FIG. 1.

In FIG. 3 a variable power source voltage generating circuit, generallyindicated VPS, and which is basically a voltage modulating circuit, isemployed for producing a power supply voltage varied in magnitude inaccordance with the input signal applied to the input, or base,electrodes of the transistors Q₁ and Q₂. In view of the fact that theamplifier AMP produces negligible distortion, the power supply voltagemay alternatively be modulated with the output voltage at the terminalt₂ connected to the single-ended, push-pull amplifier. The power sourceincludes a transformer T₁ that has a primary winding a₁ with terminalst₁₁ and t₁₂ connected to a suitable commercial power source. Thetransformer T₁ also includes a secondary winding a₂ that has a groundedcenter tap. The ends of the secondary winding a₂ are connected to arectifier RK₁ that may be considered as two full-wave rectifiers ofopposite polarity. The positive rectified voltage is vailable at theoutput terminal t₂₁ while the negative power supply voltage is availableat the output terminal t₂₂. Since the secondary winding a₂ has agrounded center tap, the positive and negative voltages as the terminalst₂₁ and t₂₂, respectively, are of equal magnitude and are both smoothedto some degree by filter capacitors between the respective outputterminals and ground.

Two transistors Q₅ and Q₆ operate as variable impedance elementsconnected to the respective output terminals t₂₁ and t₂₂ of therectifying circuit RK₁. The effect of varying the impedance of thetransistors Q₅ and Q₆ is to vary the respective positive and negativeoutput voltages of the power supply. In accordance with the fact thatthe voltage at the terminal t₂₁ is positive, the transistor Q₅ is an NPNtransistor and has its collector connected to the terminal t₂₁. In asimilar manner, the collector of the PNP transistor Q₆ is connected tothe negative power supply terminal t₂₂. The effective collector-emitterimpedance of the transistor Q₅ is controlled by an NPN transistor Q₃that has a collector load resistor R₁ which is also connected betweenthe collector and base of the transistor Q₅. The impedance of thetransistor Q₆ is controlled by a PNP transistor Q₄ that has a collectorload resistor R₂ that is also connected between the collector and baseof the transistor Q₆. The emitters of the transistors Q₃ and Q₄ areconnected to ground. The input voltage to the base of the transistor Q₃is obtained from a first comparator CMP₁, while the base voltage for thetransistor Q₄ is obtained from a similar second comparator CMP₂. Thesecomparators have a common input terminal t₄ and separate second inputterminals t₅₁ and t₅₂, respectively. The terminal t₄ is supplied with avoltage which has the same waveform as the input or output signal of theamplifier circuit AMP, and the terminals t₅₁ and t₅₂ are supplied withrelatively high frequency (as compared to the signal S₁) sawtooth wavesignals S₂ and S'₂ of opposite polarity. These pairs of signals operatein the respective comparators CMP.sub. 1 and CMP₂ to produce pulse widthmodulated signals to be applied to the bases of the transistors Q₃ andQ₄, respectively.

The emitter electrodes of the transistors Q₅ and Q₆ are grounded throughdiodes D₁ and D₂, respectively, and are connected to positive andnegative output terminals t₃₁ and t₃₂ by filters FK₁ and FK₂,respectively. The filter FK₁ consists of a series inductor L₁ and ashunt capacitor C₁, while the filter FK₂ consists of a correspondingseries inductor L₂ and shunt capacitor C₂.

The operation of the circuit in FIG. 3 will be described with referenceto the voltage waveforms shown in FIGS. 4A-4D and in FIGS. 5A-5F. It isassumed that the amplifier AMP is substantially free of distortion sothat the signal waveform S₁ in FIG. 4A may be considered to be eitherthe input or output signal of the amplifier circuit. It is the signal S₁that is applied to the common input terminal t₄ of the comparators CMP₁and CMP₂. The sawtooth voltage waveform applied to the terminal t₅₁ ofthe comparator CMP.sub. 1 is indicated by the waveform S₂ in FIG. 4A. Asimilar sawtooth wave of opposite polarity and indicated as the wave S'₂and shown in FIG. 4B is applied to the terminal t₅₂ of the secondcomparator CMP.sub. 2. As a result, the comparators CMP₁ and CMP₂produce, at their respective output terminals, pulse width modulatedsignals S₃ and S₄ in FIGS. 4C and 4D, respectively. These pulse widthmodulated signals S₃ and S₄ are applied in the proper polarities to thebase electrodes of the transistors Q₃ and Q₄, respectively, to effectthe necessary control of the transistors Q₅ and Q₆ to transmit currentpulses of varying width therethrough. Because of the smoothing effectsof the filters FK₁ and FK₂, the power source voltages at the outputterminals t₃₁ and t₃₂ have the positive and negative values V(+B) andV(-B) with waveforms as shown in FIGS. 5A and 5D, respectively. Theseare the voltages that are applied to the collectors of the transistorsQ₁ and Q₂ of the single-ended, push-pull amplifier AMP, and they areproportional to the input signal at the terminal t₁ during alternatehalf-cycles of the input signal. During the remaining half-cycles thevoltages at the terminals t₃₁ and t₃₂ are zero.

The signal output voltage at the output terminal t₂ of the amplifier AMPis assumed to have the waveform v_(o) as shown in FIGS. 5A and 5D. Themaximum excursions of the voltage v_(o) are between the maximum positivevoltage +V_(CC) and the maximum negative voltage -V_(CC) of the powersource voltages V(+B) and V(-B). The difference between the respectivevoltages V(+B) and V(-B) and the voltage v_(o) is the voltage across theconductive collector-remitter circuit of the respective transistors Q₁and Q₂ during their conductive half-cycles. Thus, the voltage across thecollector-emitter circuit of the transistor Q₁ has the value v₁ shown inFIG. 5B and the voltage across the emitter-collector circuit of thetransistor Q₂ has the value v₂ shown in FIG. 5E. As a result, collectorcurrents i₁ and i₂ with waveforms shown in FIGS. 5C and 5F flow throughthe transistors Q₁ and Q₂, respectively. It shoud be noted that when thecollector current i₁ flows through the transistor Q₁, the voltage v₁across the collector-emitter circuit of the transistor has a constant,low value, as shown in FIG. 5B. Similarly, when the collector current i₂flows through the transistor Q₂ the voltage v₂ across theemitter-collector circuit also has a constant low value, as shown inFIG. 5E. Accordingly, the power loss of the transistors Q₁ and Q₂ whenthey are conductive is much lower in the circuit in FIG. 3 than is thecase of the transistors Q₁ and Q₂ in the prior art circuit shown inFIG. 1. This is shown in the graph in FIG. 6 by a curve b, from which itis apparent that the efficiency of the amplifier circuit of theinvention shown in FIG. 3 is approximately constant at about 90% from alow output value to a high output.

FIG. 7 shows a push-pull circuit that is not of the single-endedconfiguration but has two NPN transistors Q'₁ and Q'₂. A phase-splittertransistor Q₇ is connected to an input terminal t'₁ and has collectorand emitter load resistors R₅ and R₆, respectively. The collector of thephase-splitter transistor Q₇ is connected by way of an RC couplingcircuit consisting of a capacitor C₃ and a resistor R₃ to the base ofthe transistor Q'₁. The emitter of the phase-splitter transistor Q₇ isconnected by a similar RC coupling circuit consisting of a capacitor C₄and a resistor R₄ to the base of the transistor Q'₂. The collectors ofthe transistors Q'₁ and Q'₂ are connected to opposite ends of theprimary winding b₁ of an output transformer T₂. This transformer has acenter-tapped secondary winding b₂ with end terminals t'₂ and t"₂connected across the load Z_(L). The power supply that is to have itsoutput voltage modulated in the circuit in FIG. 7 includes a rectifierRK₂, which in this case is a single diode connected to a power inputterminal t₆. There is a smoothing capacitor across the output of therectifier circuit RK₂. The output terminal t₂₁ of the rectifier circuitis connected to a circuit that is basically the same as the positivesection of the power supply in the circuit in FIG. 3, and includes theseries connected transistor Q₅ with the diode D₁ connected between theemitter of the transistor and ground. The filter circuit FK₁ consistingof the series inductor L₁ and shunt capacitor C₁ is connected betweenthe emitter of the transistor Q₅ and the power supply output terminalt₃₁.

The transistor Q₅ is controlled by the transistor Q₃ that has acollector load resistor R₁ connected to the power supply terminal t₂₁and across the collector and base electrodes of the transistor Q₅. Theoutput of a comparator circuit CMP₁ is connected to the base of thetransistor Q₃, and this comparator has two input terminals t₄ and 5₅₁.The two ends of the secondary winding b₂ of the transformer T₂ areconnected by way of a pair of diodes D₃ and D₄ to the ungrounded end ofa resistor R₇ at the terminal t₄ of the comparator CMP₁.

The operation of the circuit in FIG. 7 will be described with referenceto the waveforms in FIGS. 8A-8F. FIG. 8A shows the waveform of the inputvoltage applied to the terminal t'₁ and, in the absence of distortion,the output voltage across the load Z_(L). FIG. 8B shows the waveform ofthe power supply voltage V(B) obtained at the output terminal t₃₁ of thepower supply. This voltage has the waveform of a full-wave rectifiedsignal voltage v_(o) due to the fact that the output voltage across thecenter-tapped secondary winding b₂ of the transformer T₂ is passedthrough a full-wave rectifier D₃ and D₄ to the input terminal t₄ of thecomparator CMP₁. The push-pull amplifier transistors Q'₁ and Q'₂ aremade conductive and non-conductive alternately in accordance with theinput signal applied to the input terminal t'₁, and thecollector-emitter voltages of the transistors Q'₁ and Q'₂ have thewaveforms v₁ and v₂ shown in FIGS. 8C and 8E, respectively. Since bothof the transistors Q'₁ and Q'₂ are of the same polarity type, theyrequire the same polarity of voltage to be supplied to them to make themconductive, but because of the push-pull connection, they are conductivealternately. When the respective transistors Q'₁ and Q'₂ are conductive,so that the currents i₁ and i₂ (FIGS. 8A and 8F) flow therethrough, thevoltages v₁ and v₂ are constant and are equal to the low voltage acrossthe collector-emitter circuits of the respective transistors Q'₁ andQ'₂. Therefore, the transistors Q'₁ and Q'₂ consume less power andoperate at greater efficiency than they would if supplied with a fixedpower supply voltage in accordance with the prior art.

FIG. 9 shows a single-ended amplifier connected to obtain the advantageof a modulated power supply voltage in accordance with the presentinvention. The amplifier in FIG. 9 includes a transistor Q₈ operated asa class A emitter-follower amplifier. The transistor Q₈ has an inputterminal t₁ connected to its base and an emitter resistor R₈. Theemitter is connected through a capacitor C₅ to the output terminal t₂and the load Z_(L) is connected between the terminal t₂ and ground.

The power supply in FIG. 9 is quite similar to that in FIG. 7. There isa direct, rather than a rectifier, connection from the emitter of thetransistor Q₈ to the input terminal t₄ of the comparator CMP₁. Thepreviously mentioned sawtooth wave signal is connected to a second inputterminal t₅₁ of the comparator CMP₁. The pulse width modulated signalobtained at the output of the comparator is connected to the base of thecontrol transistor Q₃. The remainder of the power supply circuit is thesame as in FIG. 7 and need not be described again.

The operation of the circuit in FIG. 9 will be described with thereference to FIGS. 10A-10D. FIG. 10A shows the waveform of the outputvoltage v'_(o) obtained at the emitter electrode of the transistor Q₈.The modulated waveform of the source voltage V(B) corresponds to thewaveform of the voltage v'_(o) and is higher than the latter by theamount of the voltage drop across the collector-emitter circuit of theamplifier transistor Q₈. The peak value of the voltage V(B) is theoriginal power supply voltage +V_(CC).

FIG. 10B shows the waveform of the output voltage v_(o) after its directvoltage component has been eliminated by the capacitor C₅. The voltagev_(o) in FIG. 10B is the voltage between the output terminal t₂ andground. The source voltage V(B) shown in FIG. 10A and applied to thecollector of the transistor Q₈ is modulated, or varied, in response tothe output voltage v'_(o), so that a voltage v of low, constantamplitude, as shown in FIG. 10C, is present across the collector-emittercircuit of the transistor Q₈. Thus, even though a current i having awaveform as shown in FIG. 10D passes through the collector-emittercircuit of the transistor Q₈, the power consumption of this transistoris reduced with respect to that of the standard class A, single-endedamplifier, and its efficiency is increased by virtue of the low voltageacross the collector-emitter circuit.

With the amplifier circuits of the invention as described, since thepower supply source voltage, which is changed in accordance with theinput signal applied to the input electrode of the amplifier is appliedthrough the load of the amplifier across the output electrode and commonelectrode thereof, the voltage applied across the output electrode andcommon eletrode of the amplifier is relatively low, and therefore thepower consumption of the amplifier is relatively low. Amplitudemodulation of the power supply voltage in accordance with the inputsignal or output signal of the amplifier prevents an excess voltage frombeing applied to the amplifier, so that efficiency of the amplifier isimproved. This efficiency is further improved in the case of a push-pullamplifier.

In the examples of the invention as shown in the drawings, a bipolartransistor is used as the amplifying element or elements, but it is alsopossible to use field effect transistors, vacuum tubes, or the like asthe amplifying elements.

In addition, the present invention can be utilized in various types ofamplifier circuits such as class A, class B, class AB, or single-endedand push-pull types. In push-pull amplifiers, it is possible to usetransistor amplifying elements of the same conductivity type or ofcomplementary conductivity types.

It will be apparent that many modifications and variations can beeffected in the embodiments shown without departing from the true scopeof the invention as defined by the following claims.

What is claimed is:
 1. In an amplifier having signal input meanssupplied with an input signal to be amplified, signal output meansconnected with a load, amplifying means connected between said signalinput means and said signal output means for supplying an output signalto the latter, and power supply means for supplying an operating voltageto said amplifying means; a circuit for modulating said operatingvoltage supplied to the amplifying means in accordance with one of saidinput and output signals comprising direct voltage source means, a firsttransistor having base, emitter and collector electrodes, smoothingmeans connected in series with the collector-emitter circuit of saidfirst transistor between said direct voltage source means and saidamplifying means, a first resistor connected between the base andcollector electrodes of said first transistor, a second transistorhaving base, emitter and collector electrodes, the collector-emittercircuit of said second transistor being connected between the baseelectrode of said first transistor and a reference voltage point, anamplitude comparator, and means for applying to said comparator asawtooth wave signal and a signal varying with said one of the input andoutput signals so that said comparator provides a pulse width modulatedsignal having a duty factor varying with said one signal and which isapplied to said base electrode of said second transistor.
 2. Anamplifier according to claim 1, in which said amplifying means comprisesa further transistor having base, emitter and collector electrodes, thecollector-emitter circuit of said further transistor being connected inseries with said load between said smoothing means and the referencevoltage point, the base electrode of said further transistor beingconnected to said signal input means and said load being connected tosaid comparator to supply the output voltage across said load to saidcomparator for comparison with said sawtooth wave signal.
 3. Anamplifier according to claim 2, further comprising means to bias saidfurther transistor to operate in the class A mode.
 4. An amplifieraccording to claim 1, in which said direct voltage source means haspositive and negative voltage terminals, said amplifying means is of thepush-pull type and has first and second operating voltage supplyterminals, and said smoothing means and said collector-emitter circuitof the first transistor are connected in series between said positivevoltage terminal and said first operating voltage supply terminal; andsaid circuit for modulating the operating voltage supplied to theamplifying means further includes a third transistor having base,emitter and collector electrodes, second smoothing means connected inseries with the collector-emitter circuit of said third transistorbetween said negative voltage terminal and said second operating voltagesupply terminal, a second resistor connected between the base andcollector electrodes of said third transistor, a fourth transistorhaving base, emitter and collector electrodes, the collector-emittercircuit of said fourth transistor being connected between the baseelectrode of said third transistor and said reference voltage point, asecond amplitude comparator, and means for applying to said secondcomparator a second sawtooth wave signal of opposite polarity to thefirst mentioned sawtooth wave signal and said signal varying with saidone of the input and output signals so that said second comparatorprovides a second pulse width modulated signal having a duty factor alsovarying with said one signal and which is applied to said base electrodeof said fourth transistor, with said second pulse width modulated signaland the first mentioned pulse width modulated signal occurringalternatively.
 5. An amplifier according to claim 4, in which the firstmentioned smoothing means and said second smoothing means areconstituted by first and second low pass filters, respectively, andfirst and second diodes are connected between said reference voltagepoint and connection points of the emitter electrode of said firsttransistor with said first low pass filter and of the emitter electrodeof said third transistor with said second low pass filter, respectively,with the polarities of said first and second diodes being the same asthe base-emitter junctions of said first and third transistors,respectively.
 6. An amplifier according to claim 4, in which saidpush-pull amplifying means includes a pair of additional transistorsconnected in complementary push-pull relationship.
 7. An amplifieraccording to claim 6, in which said pair of additional transistors eachhave base, emitter and collector electrodes, the collector electrodes ofsaid additional transistors are respectively connected to said first andsecond operating voltage supply terminals, the emitter electrodes ofsaid additional transistors are connected to each other and to saidoutput terminal means, the load is connected in series between saidoutput terminal means and said reference voltage point, and said baseelectrodes of the additional transistors are connected with said inputterminal so as to be supplied with the input signal with the samepolarity.
 8. An amplifier according to claim 1, in which said amplifyingmeans is of the push-pull type and includes third and fourth transistorsof the same conductivity type each having base, emitter and collectorelectrodes, and said signal input means includes phase splitting meansreceiving said input signal and applying input signals of oppositepolarity to the base electrodes of said third and fourth transistors. 9.An amplifier according to claim 8, in which said signal output meanscomprises a signal output transformer having primary and secondarywindings, the primary winding being center tapped and the ends thereofbeing connected to the collector electrodes of said third and fourthtransistors, the center tap of said primary winding being supplied withthe output of said smoothing means, and said secondary winding beingconnected across the load.
 10. An amplifier according to claim 9, inwhich said means for supplying to said comparator a signal varying withone of said input and output signals includes a full wave rectifierconnected across said load and to said comparator for supplying to thelatter a full wave rectified signal of the voltage across the load.